Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gases. Vehicles equipped with diesel or lean gasoline engines offer the benefits of increased fuel economy. Such vehicles are typically equipped with lean exhaust gas aftertreatment devices, such as, for example, Active Lean NOx (ALNC) Catalysts, which are capable of continuously reducing NOx emissions in an oxygen rich environment. In order to maximize NOx reduction in the ALNC, a hydrocarbon-based reductant, such as fuel, is introduced into the exhaust gas entering the device via a reductant injection system. Once the ALNC has reached certain operating temperatures, further injection of reductant will create an exothermic reaction in the ALNC and thus cause its temperature to rise. As the ALNC ages, its efficiency reduces. The reduction in efficiency results in a reduction of a temperature rise, or exotherm, across the ALNC as compared to an exotherm for a non-degraded catalyst. It is therefore possible to diagnose degradation of the ALNC by monitoring the exotherm generated as a result of hydrocarbon combustion.
One such system is described in U.S. Pat. No. 5,938,715, wherein catalytic exotherm is calculated using a thermodynamic model of the catalyst. The calculated exotherm is then compared to an expected exotherm value in order to determine conversion efficiency of the catalyst.
The inventors herein have recognized a disadvantage with the prior art approach. Namely, prior art teaches modeling catalyst behavior only at steady state, therefore any variations due to transient conditions are not accounted for. Further, a typical ALNC catalyst has a certain amount of hydrocarbon storage capacity. Hydrocarbons in the exhaust gas entering the ALNC are stored at lower temperatures, and are released as ALNC temperature increases. The released hydrocarbons combust in the ALNC and can cause temperature elevations in addition to the elevation caused by external reductant injection. Therefore, the inventors have determined that it is crucial to account for hydrocarbon storage and release mechanisms in the ALNC in order to accurately estimate the exotherm across the ALNC. Further, as the catalyst ages, its hydrocarbon conversion efficiency degrades. Thus, it is also important that the exotherm calculations account for reduced hydrocarbon conversion efficiency due to ageing.
Additionally, the inventors have recognized that at some operating conditions, such as at engine idle, the magnitude of the exotherm is so small, that in order to minimize the effect of various noise factors on the diagnostic accuracy, the exotherm may need to be increased through extra reductant injection.